Ch 5 · Noble Gases · O Level Chemistry 5070
Topic 5 of 5 in The Periodic Table — O Level Chemistry 5070 — 38 lessons.
Introduction
Almost one per cent of the air you breathe is argon, yet it plays no part in respiration, burning or any other reaction: it is a noble gas. Group VIII (also written Group 0) is the only group whose defining property is doing nothing. This short lesson covers why the noble gases are unreactive, what that makes them useful for, and the one trend you must be able to explain.
Key ideas — Unreactive, monatomic gases
The noble gases are helium, neon, argon, krypton, xenon and radon. All are colourless gases at room temperature and all are monatomic: they exist as single atoms, not molecules, so their formulae are simply He, Ne, Ar and so on.
🧑🏫 Teacher's explanation
Imagine you are at a party where everyone is trying to find a partner to dance with. Most people (like Sodium or Chlorine) are constantly moving around, trying to find someone to hold hands with (covalent bonding) or someone to swap belongings with (ionic bonding) just to feel 'complete'. However, the Noble Gases are like the most popular, self-sufficient VIPs at the party. They already have everything they need; they are perfectly happy standing alone without needing to hold hands or swap anything. Because they are so 'satisfied' with their current state, they don't bother interacting with anyone else.
📖 Definitions
🔍 Worked example — Easy
Explain why Neon (Ne, atomic number 10) is unreactive using its electronic configuration.
🔍 Worked example — Standard
Compare the boiling points of Helium (He) and Argon (Ar). Which has a higher boiling point and why?
🔍 Worked example — Exam-style
A sample of Argon gas is heated. If the mass of the sample is 40 g, calculate the number of moles present. (A_r of Ar = 40).
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Uses of the noble gases
Their uses all depend on their lack of reactivity, their low density or the light they give when electricity passes through them.
🧑🏫 Teacher's explanation
Imagine you are at a crowded party. Most people (the reactive elements) are constantly trying to shake hands, swap business cards, or start arguments (forming chemical bonds) to feel 'complete'. The noble gases, however, are the people standing alone in the corner with their arms folded, perfectly content and having no interest in interacting with anyone else. Because they already have a full outer shell of electrons, they are 'satisfied' and won't react.
📖 Definitions
🔍 Worked example — Easy
Explain why helium is used in party balloons instead of hydrogen, considering both physical and safety properties.
🔍 Worked example — Standard
A student observes that the boiling point of noble gases increases as you move down Group VIII. Predict the trend in boiling point from Helium to Xenon.
🔍 Worked example — Exam-style
Calculate the mass of argon gas required to fill a container if the container holds 0.5 moles of argon. (A_r of Ar = 40).
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(b) State one use for argon in industrial processes. [1] [3 marks]
Key ideas — Trend in boiling point
The noble gases have very low boiling points, but the boiling point increases steadily down the group:
🧑🏫 Teacher's explanation
Imagine you are trying to pull two magnets apart. If the magnets are tiny and weak, they slide away from each other with almost no effort. If the magnets are large and heavy, you have to pull much harder to separate them. Noble gases behave similarly. Even though they are single atoms (monatomic), they still experience very weak 'sticky' forces between them.
📖 Definitions
🔍 Worked example — Easy
Explain why the boiling point of Xenon (M_r = 131) is higher than that of Helium (M_r = 4).
🔍 Worked example — Standard
Predict whether the boiling point of Radon (M_r = 222) is higher or lower than that of Xenon (M_r = 131).
🔍 Worked example — Exam-style
A sample of Argon is cooled until it liquefies. If the boiling point is -186 °C, what is the temperature in Kelvin?
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Explain why the boiling point increases as you move from Helium to Argon. [3 marks] (b) Predict whether the boiling point of Krypton (M_r = 84) will be higher or lower than Argon. [1 mark] [4 marks]
Worked example
Question. (a) Explain why argon exists as single atoms and does not form compounds. [3] (b) Explain why argon rather than air is used to fill filament lamps. [2] (c) Use the data in the table above to predict the boiling point of radon and explain the trend. [3]
🔍 Worked example — Easy
An atom of Neon has an electronic configuration of 2, 8. Explain why Neon is unreactive and exists as a monatomic gas.
🔍 Worked example — Standard
Predict the boiling point trend for Group VIII elements and explain why the boiling point increases from Helium to Xenon.
🔍 Worked example — Exam-style
A lamp uses Argon instead of air to prevent the filament from oxidizing. Explain why Argon is suitable for this purpose.
⚠️ Common errors
Common mistakes
Practice
Answers Gases [B1]; monatomic, existing as single atoms [B1]. Their atoms have full outer electron shells [B1], so they have no tendency to lose, gain or share electrons and do not form bonds [B1]. Helium in balloons or airships because it has a low density and is non-flammable [B2]; argon in filament lamps because it is inert so the filament does not burn away [B2]. Krypton atoms have a larger relative atomic mass [B1], so the attractive forces between atoms are stronger and more energy is needed to separate them [B1].